A groundwater production well (also sometimes referred to herein as a “groundwater well”, a “production well”, or simply as a “well”) is a structure where groundwater is produced for consumption by people, for animal livestock, and for agricultural purposes, as well as industrial purposes (such as refining, mining, landfills, technology and so forth). Groundwater production wells can also include test holes for groundwater exploration. These wells consist of a support casing and well screen, through which groundwater enters the well. These wells may also be constructed in bedrock and serve the same purpose. There is also a primary pump inside the well, typically consisting of a line shaft turbine or electric submersible pump that is positioned at depth inside the well. Typically, the pump diameters are large relative to the size of the casing and well screen as well as the pump column that extends between the pump and the ground surface. Moreover, each section of pump column is connected by means of a larger diameter threaded collar. Therefore, the pump column consists of ten to twenty foot sections of pipe of a smaller diameter but terminated on each end by a collar that is at least one-half inch to one inch larger than the main section of the pipe itself.
Dynamic flow profiling of wells has been common practice for numerous years. Such flow profiling of these production wells, with various types of down-hole devices and instruments (flowmeters), is typically undertaken soon after construction of a well is completed or when a specific capacity problem arises at some point in the well's aging process. The flow profile shows the zonal flow contribution along the length of the well screen. For any new well, the flow profile will look different depending on the pumping rate, the depth location of the pump intake, and the geological properties of the surrounding formational materials (such as permeability, angularity, fracture and fault structures, etc.), as well as a result of hydraulic interferences from pumping wells located in close proximity of the well where the flow meter survey is being performed. Well aging can also affect the morphological manifestation of the flow distribution profile. This is because wells typically become increasingly clogged over time from buildup of mineralization deposits, sediment infill and biological matter inside the well screen and surrounding gravel pack, to the degree that specific capacity and zonal flow contribution of the well changes and decreases over time.
Specific capacity (SC) is defined by the number of gallons per minute (GPM) that the well produces per foot of drawdown inside the well when it is pumping at a constant rate. For example, when the well is pumping at 1,000 GPM with 50 feet of drawdown inside the well, the SC of the well is equal to 20 GPM/foot drawdown—or simply referred to as SC=20. As the well ages, for example, the SC may incrementally decrease to 4; meaning that there is 250 feet of drawdown of the pumping water level to maintain the pumping rate at 1,000 GPM. This type of low SC value can be alarming in terms of the financial bottom line of the producer since the lower the SC, the greater the consumption of electricity due to an increase in total dynamic head (TDH). TDH is equal to the vertical distance between the pumping water level and the ground surface.
When such decreases in SC are found within an aging well, it can be desired to attempt to rehabilitate the well. The intention of the rehabilitation process is to restore as much SC to the well as possible. However, the reality is that SC is never completely restored, but can only be recovered to a percentage of the original value. While a decrease in drawdown during pumping at a constant rate is one metric of the well's SC improvement from the rehabilitation process, another metric is the improvement in zonal yield (ZY) and zonal specific capacity (ZSC). The most tried and true method of measuring the improvement in ZY and ZSC is through performance of the down-hole flow metering survey inside the well, along the entire length of the well screen. Performing the flow metering survey on the basis of just before and just after the rehabilitation process is very useful for measuring the zonal improvements, section by section along the length of the well screen.
The difference between the ZY and ZSC before and after the rehabilitation process is an important metric by which the producer can gauge the effectiveness of the rehabilitation being performed. Thus, such a comparison can be used as a means of evaluating contractual performance objectives for the company that has been contracted to perform the rehabilitation. Some rehabilitations are relatively simple and inexpensive, such as rehabilitations merely consisting of mechanical wire brushing of the inside of the well. However, other types of rehabilitation problems can be much more complex and costly when significant clogging of the gravel pack is involved. Gravel pack clogging problems often require acoustical and/or chemical treatments on a single or even repeat basis before reaching a rehabilitation performance level where the SC is sustainable over a prolonged period of time (typically measured in years). However, the interpretation of the before and after flow metering survey results can be problematic as a result of internal well roughness and high velocity water jets entering the well, thus overprinting aberrant water-jet velocities onto the normal pumping velocity gradient inside the well. These aberrant velocities often lead to (falsely) negative flow results implying that groundwater is actually leaving the well in certain zones during constant rate pumping.
To make matters even more complex and riddled with error, the flow metering surveys performed inside wells are typically not centralized within the well. Instead, it is a typical practice to insert the flow metering instrument along one side of the well. This can be done through either an external camera tube that enters the well below the pump intake, or through a large diameter PVC pass through pipe inside the well that runs alongside the pump column, bowls and intake, with an entry point somewhere below the pump intake and above the well screen. A common tool used for flow profiling is a spinner log tool. However, when an internal, large diameter (e.g., two inches to four inches in diameter) PVC pipe is used to allow access of a spinner log tool into the well, the primary pump cannot be used for the spinner log survey, since there is not enough diameter inside the well to accommodate both the PVC pipe and the primary pump. Therefore, an added expense to the process of the flow metering survey is for the potential use of a test pump, the rental of which from a local pump service company can typically cost tens of thousands of dollars.
In either entry process described above, the spinner log tool typically begins at a decentralized starting point on one side of the well and often gradually shifts to the other side of the well (and even back again to the original side) during its survey descent. Tool drift stems from the fact that in almost every case, wells are not plumb (i.e. precisely vertical), but instead deviate from plumb by two or more degrees. Therefore, the decentralized survey is being conducted through varying flow regimes inside the well commonly referred to as the boundary layer (i.e. near or adjacent to the support casing or well screen), the transitional zone, and the axial trace (i.e. toward or at the center of the well) of fluid flow. The fluid velocity in each zone is different, with the boundary layer producing the slowest velocities and the axial trace producing the fastest velocities.
In order to avoid measuring varying velocity regimes inside of a production well, the spinner log tool can be deliberately centralized using a centralizer device whereby the spinner log tool follows a fixed axial path inside the well. So, although the device is only measuring the fastest in-well velocities (i.e. in the axial trace), correction factors can be used to derive the bulk average flow rate, since the error is a constant. These correction factors cannot be used when the tool is drifting from one side to the other side of the well since the error is not a constant resulting from tool drift in the x-y cross-sectional plane of the well. The level of error that can result from tool drift can be as much as fifty percent (50%) in terms of underestimating or overestimating zonal flow.
All of the factors that contribute to these measurement errors can cause large uncertainties in whether or not the data is reliable and acceptable for use as a performance metric for measuring pre-rehabilitation and post-rehabilitation zonal flow results. But, the fact of the matter is that the dynamic, steady-state flow metering tests must be performed with a pump inside the well, which leads to the decentralized position of the spinner log or other types of flow metering devices. To deal with such decentralization problems, spring-loaded centralizers have been developed, whereby the centralizer can be triggered to expand once the device is inside the well and below the pump intake. Supposedly, these devices can also be remotely retracted in order to remove the device. However, the situation happens often enough where the centralizer does not either expand or collapse as desired. In the case of a camera tube entry scenario and where the spring-loaded centralizer does not collapse, the flow metering tool can then easily get stuck inside the well, which can result in serious cost ramifications. As an example, the camera tube may become compromised to the point where it can no longer be used for video inspection of the inside of the well. Such video inspections are useful in conjunction with SC measurements to determine when a well should be rehabilitated. However, if the camera tube is no longer accessible, the primary pump must be removed before the camera survey can commence. Given the risk of compromising the functionality of the camera tube, the spring-loaded centralizers are typically not used and the decentralized flow metering results considered to be good enough.
Another application of flow profiling of pumping production wells is the use of such data in combination with depth-dependent, co-located water chemistry samples whereby the cumulative results can be flow-weighted through integration of the depth-dependent cumulative flow data. This algebraic transformation provides the zonal chemical distribution for any analyte or water characteristic of concern in association with the volume of groundwater produced from each of these zones. The benefit of this application is for purposes of modifying the internal fluid entry hydraulics of the well to inhibit groundwater of undesirable water quality from entering the well from specific formational zones. These manipulations can therefore reduce the amount of contaminant mass discharging from the well, such that no or a reduced amount of treatment and/or blending of the water from the well may be required over time.
In each application explained above (i.e. flow only and flow combined with water quality), there is significant room for error and costly mistakes from the flow meter results that are a subject of concern and confusion for producers and the hydrogeological and engineering professionals that provide (potentially low confidence) interpretation and meaning of such data. It is important to emphasize that the goal of the flow metering survey under both dynamic (pumping) and ambient (non-pumping) steady-state conditions is to determine a reliable value of the bulk average flow rate at any point inside the well. Such data then leads to credible performance metric evaluations for rehabilitation, as well as success solutions for inhibiting groundwater of undesirable water quality from entering the well.